Alkylated aromatics serve as the foundation for producing a variety of useful petrochemicals. For instance, ethylbenzene, cumene, ethyltoluene, and isopropyl toluene (cymene), as well as other alkyl-substituted aromatics, are beneficial as feedstocks for the production of a variety of styrenic and phenolic polymer materials. Alkyl-substituted aromatics also are useful as high octane transportation fuels. The use of alkyl-substituted aromatics as blending agents for gasoline expands product volume and increases octane values. Aromatic alkylation processes also provide an economic method of reducing benzene content in gasoline.
Most current processes for producing alkylated aromatics require the use of substantially pure olefins as alkylating agents. More economical sources are needed for olefins, such as ethylene and propylene, which are relatively expensive to produce in substantially pure form.
Cheap sources of ethylene and propylene include refinery off-gases or tail gases, which contain dilute concentrations of both ethylene and propylene, and possibly higher olefins. An even more common source of a mixed ethylene/propylene feedstock is the effluent produced by the thermal cracking of hydrocarbon streams in olefin production operations. Such a cracking product generally is much higher in mixed olefin content than are refinery off-gases.
It would be economically desirable to use these refinery gas streams as alkylating agents to produce alkylated aromatics. Unfortunately, when a mixed olefin gas stream is used to alkylate aromatic compounds, a mixture of several types of alkylated aromatic products is produced. The mixed product typically includes both ethylbenzene and cumene, which are difficult to separate.
Economical methods are needed to use these dilute mixed olefin streams to selectively produce cumene in one stage and ethylbenzene in a second stage. Several U.S. patents suggest the use of a mixed ethylene/propylene stream to alkylate benzene, namely, U.S. Pat. No. 4,891,458 and U.S. Pat. No. 4,447,664. However, neither of these patents teaches or suggests a method in which cumene is selectively produced in a first stage and ethylbenzene is produced in a second stage.
U.S. Pat. No. 4,447,664 describes a method to alkylate aromatic hydrocarbons using as an alkylating agent a product stream from a Fischer-Tropsch reaction which contains dilute amounts of both ethylene and propylene. The patent teaches that the "selected alkylating agent" can be an "ethylene/propylene" mixture. The patent also teaches that, prior to the alkylation of the aromatic material, alkylating agents other than the selected alkylating agent must be substantially removed from the stream, preferably by distillation. A method besides distillation is given for removing propylene when propylene is not the selected alkylating agent. That method involves the conversion of propylene and higher olefins to fuel oil and gasoline, as described in U.S. Pat. No. 4,227,992. U.S. Pat. No. 4,447,664 does not teach a process in which propylene is selectively reacted out of the stream to produce cumene in a first stage and thereafter the remaining propylene-depleted stream comprising ethylene may be reacted with the aromatic in a second stage to produce ethylbenzene.
U.S. Pat. No. 4,891,458 also teaches that mixtures of olefins containing 2 to 4 carbon atoms can be used to alkylate the aromatic hydrocarbons. However, when referring more specifically to the production of cumene, the patent admits that only a small fraction of by-product ethylbenzene and n-propylbenzene can be economically removed by distillation. The patent concludes that the feedstock must contain very little ethylene and that a catalyst should be used which makes very little of these impurities.
U.S. Pat. No. 4,387,259 describes the use of an intermediate pore size zeolite--"ZSM-12"--to catalyze a reaction in which a mixed ethylene/propylene stream is reacted with a stream containing benzene to produce cumene but not ethylbenzene. A need exists to identify additional catalysts and catalyst systems for the selective production of cumene and, if desired, ethylbenzene, using readily available, low cost mixed ethylene/propylene streams as the alkylating agent.